Flame holders with fuel and oxidant recirculation, combustion systems including such flame holders, and related methods

ABSTRACT

Embodiments disclosed herein include flame holders that may provide recirculated fuel flow therethrough, combustion systems that include such flame holders, and related methods. A fuel and/or fuel-oxidant mixture may pass through one or more openings in the flame holder and, after combustion, the resulting flame may be held at or near a surface of the flame holder including in the one or more openings. Generally, the configuration of the flame holders disclosed herein (e.g., the one or more openings of the flame holders) may recirculate or regulate (e.g., decrease and/or increase) the flow of fuel and/or oxidant therethrough, at least limit flame flashback, improve fuel/oxidant mixing, increase flame stability, regulate where the flame is located in the flame holder, improve the operational stability window of the combustion system, or combinations of the foregoing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Entry under 35 U.S.C. 371 of International Application No. PCT/US2015/000219 filed on 23 Dec. 2015, which claims priority to U.S. Provisional Application No. 62/096,612 filed on 24 Dec. 2014, the disclosures of which are incorporated herein, in their entirety, by this reference.

BACKGROUND

Flame stabilization can be dependent upon a speed at which a fuel-air mixture enters a flame holder where propagation of a flame is desired. For example, fuel injection at high speed toward a flame holder may contribute to or cause instabilities in flame position and shape of a flame.

High fuel injection speeds can result in non-uniform fuel distribution, insufficient mixing of the fuel with an oxidant, and unstable flame propagation within the flame holder, which can cause such problems such as poor combustion (e.g., a low percentage of the fuel oxidant mixture is combusted), failure to combust leaner fuels (e.g., low fuel to oxidant mixture), increased emissions of pollutants, combustion outside of the flame holder, poor heat transfer, reduced component life, and potential system damages among others. Additionally, fuel injection at low speeds into the flame holder may cause a flashback that can damage structures within the fuel-oxidant mixing region of the combustion system.

Therefore, developers and users of combustion systems continue to seek improvements to the flame holders.

SUMMARY

Embodiments disclosed herein include flame holders that can provide fuel and oxidant recirculation, combustion systems that include such flame holders, and related methods. A fuel and/or fuel-oxidant mixture may pass through one or more openings in a flame holder and, after combustion, the resulting flame may be held inside the flame holder and/or at or near a surface of the flame holder. Generally, the configuration of the flame holders disclosed herein (e.g., the one or more openings of the flame holders) may recirculate and/or regulate (e.g., decrease and/or increase) the flow of fuel and/or oxidant therethrough, at least limit flame flashback, improve fuel/oxidant mixing, increase flame stability, regulate where the flame is located in the flame holder, improve the operational stability window of the combustion system, or combinations of the foregoing. The openings may be shaped to promote vortex formation in the fuel, oxidant, and flame; with the vortex or vortices providing the recirculation.

In an embodiment, a flame holder is disclosed. The flame holder includes a refractory body defining a proximal side, a distal side spaced downstream from the proximal side, and a plurality of openings extending therethrough. Each of the plurality of openings extends between an inlet on the proximal side and an outlet on the distal side. The inlet has a first area in plan view. Each of the plurality of openings includes a bulging region located downstream from the inlet thereof. The bulging region has a second area in plan view that is greater than the first area.

In an embodiment, a combustion system is disclosed. The combustion system includes a flame holder. The flame holder includes a refractory body defining a plurality of openings therein. Each of the plurality openings includes an inlet and a bulging region located downstream from the inlet. The flame holder also includes one or more nozzles configured to dispense fuel toward the flame holder and into the plurality of openings thereof. The flame holder further includes one or more flame holder supports supporting the flame holder above the one or more nozzles to define a standoff between the one or more nozzles and the flame holder.

In an embodiment, a method of operating a combustion system is disclosed. The method includes dispensing fuel from one or more nozzles toward inlets of a plurality of openings of a flame holder and further toward outlets of the plurality of openings of the flame holder. The method also includes decreasing a velocity of the fuel as the fuel passes through at least a portion of the plurality of openings. Finally, the method includes igniting at least a portion of the fuel in the flame holder.

Features from any of the disclosed embodiments may be used in combination with one another, without limitation. In addition, other features and advantages of the present disclosure will become apparent to those of ordinary skill in the art through consideration of the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate several embodiments, wherein identical reference numerals refer to identical or similar elements or features in different views or embodiments shown in the drawings.

FIG. 1A is an isometric view of a combustion system according to an embodiment.

FIG. 1B is an isometric cutaway view of the combustion system shown in FIG. 1A taken along plan 1B-1B.

FIGS. 2A-2D are partial cross-sectional views of flame holders having at least one opening therein, according to different embodiments.

FIG. 3 is an isometric cutaway view of a combustion system, according to an embodiment.

FIGS. 4A-4F are partial cross-sectional views of flame holders, according to different embodiments.

FIG. 5 is a partial cross-sectional view of a flame holder defining an opening, according to an embodiment.

FIGS. 6A and 6B are partially cross-sectional views of flame holders that include one or more mechanisms disposed in an opening thereof that increase mixing of a fuel/oxidant, according to an embodiment.

FIG. 7 is a partial cross-sectional view of a flame holder including a sleeve that is attachable to a refractory plate, according to an embodiment.

FIGS. 8A-8D are top plan views of a distal side of flame holders, according to different embodiments.

FIG. 9 is a top plan view of a flame holder assembly that includes a plurality of flame holders, according to an embodiment.

DETAILED DESCRIPTION

Embodiments disclosed herein include flame holders that may provide fuel and oxidant recirculation, combustion systems that include such flame holders, and related methods. A fuel and/or fuel-oxidant mixture may pass through one or more openings in a flame holder and, after combustion, the resulting flame may be held at or near a surface of the flame holder including in the one or more openings. Generally, the configuration of the flame holders disclosed herein (e.g., the one or more openings of the flame holders) may recirculate and/or regulate (e.g., decrease and/or increase) the flow of fuel and/or oxidant therethrough, at least limit flame flashback, improve fuel/oxidant mixing, increase flame stability, regulate where the flame is located in the flame holder, improve the operational stability window of the combustion system, or combinations of the foregoing. The openings may be shaped to promote vortex formation in the fuel, oxidant, and flame; with the vortex or vortices providing the recirculation.

For example, openings of the flame holder may recirculate and/or regulate flow of the fuel, oxidant, fuel-oxidant mixture, or combinations thereof (any of which may be referred to as “fuel/oxidant” herein for convenience) flowing through the openings in the flame holder. In an embodiment, the flame holder may reduce the velocity of the fuel/oxidant flowing through the openings. For example, the flame holder may initially decrease the velocity of the fuel/oxidant flowing through the openings from a first location (e.g., the inlet 118 of FIG. 1B, the narrowed region 576 of FIG. 5) toward and/or to a second location (e.g., the outlet 120 of FIG. 1B, the bulging region 322 of FIG. 3). Subsequently, the flame holder may increase the velocity thereof as the fuel/oxidant from the second location (e.g., the bulging region 322 of FIG. 3) flows towards and/or to an outlet of the openings in the flame holder. In some embodiments, the flame holder may cause the fuel and/or oxidant to initially exhibit a relatively high velocity at and/or near an inlet of the openings to thereby substantially prevent flashback. In some embodiments, the flame holder may cause the fuel/oxidant to form eddies or vortices therein that increase flame stability. In some embodiments, the flame holder may regulate a velocity of the fuel/oxidant therein, thereby regulating where flame is combusted in the flame holder. In some embodiments, the flame holder may trap the flame in the opening and/or improving the mixing of the fuel and/or oxidant. In some embodiments, regulating and/or modulating fuel/oxidant through the openings may improve flame stabilization, flame robustness, improve heating efficiency, reduce NO_(x) formation, improve mixing of fuel and oxidant, prevent flashback, or combinations of the foregoing.

FIGS. 1A and 1B are isometric and isometric cutaway views, respectively, of a combustion system 100, according to an embodiment. The combustion system 100 includes a flame holder 102 and one or more nozzles 106 spaced from and oriented toward the flame holder 102. The flame holder 102 includes a plurality of openings 104 therein. The nozzles 106 may dispense a fuel/oxidant, which may generally flow toward the flame holder 102 and/or through the openings 104 of the flame holder 102.

In an embodiment, the flame holder 102 may be supported above the nozzles 106 by one or more elements or components. For example, flame holder supports 108 and/or an optional burner tile 110 may support and/or secure the flame holder 102 at a suitable distance above the nozzles 106. Generally, the burner tile 110 may encircle a combustion air passage 112 (e.g., the burner tile 110 may at least partially surround at least some of the nozzles 106). For instance, the burner tile 110 may have an approximately conical (e.g., a truncated cone) or cylindrical shape, and the encircled space of the burner tile 110 may define the combustion air passage 112. It should be appreciated that the burner tile 110 may have any suitable shape (e.g., cross-sectional shape) and/or size, which may vary from one embodiment to the next. For example, the burner tile 110 may have a rectangular, square, triangular, irregular, or any other suitable cross-sectional shape. In some embodiments, the burner tile 110 is omitted.

Furthermore, the nozzles 106 may be positioned at any number of suitable locations and/or in any number of suitable arrangements, which may vary from one embodiment to the next. In some embodiments, one, some, or all of the nozzles 106 may be located in the combustion air passage 112. Additionally or alternatively, one, some, or all of the nozzles 106 may be located outside of the combustion air passage 112 (e.g., arranged about the perimeter or periphery of the burner tile 110).

The nozzles 106 and the flame holder 102 may be arranged to provide at least partial premixing of the dispensed fuel with an oxidant (e.g., air or flue gas) in a premixing region between the nozzles 106 and the flame holder 102. In one or more embodiments, the fuel and an oxidant may mix or at least partially mix in the combustion air passage 112. For example, the fuel exiting nozzles 106 may mix with an oxidant in the combustion air passage 112. As such, the fuel at least partially mixed with an oxidant may flow toward the flame holder 102 and/or into the openings 104 therein. The fuel/oxidant may be at least partially ignited (e.g., in the openings 104 of the flame holder 102 and/or above the flame holder 102).

In any event, as shown in FIG. 1B, the nozzles 106 may be located near and/or inside the burner tile 110. In an embodiment, the burner tile 110 may support an intermediate flame between the flame holder 102 and the nozzles 106 (e.g., between an upper surface 113 of the burner tile 110 and a proximal side 114 of the flame holder 102) to pre-heat the flame holder 102 prior to transfer of combustion to the flame holder 102. For example, the upper surface 113 of the burner tile 110 may be configured to support the flame (not shown) during at least one of start-up, low fuel flow, or ignition by a pilot flame.

Generally, the flame holder 102 may include and/or be formed from any number of suitable high-temperature resistant materials, such as a refractory material as discussed in more detail below. Also, as described below in more detail, the flame holder 102 may have any suitable shape and/or size. For instance, a periphery or perimeter of the flame holder 102 may have a generally circular shape. Alternatively, the flame holder 102 may be, in plan view, generally rectangular, generally triangular, etc.

The flame holder 102 may be partially or completely formed from a refractory plate 115 or other type of refractory body. For example, the refractory plate 115 may define a proximal side 114 and a distal side 116 spaced downstream from the proximal side 114. The refractory plate 115 may have generally a plate-like shape. For example, the proximal side 114 and/or the distal side 116 may be approximately planar. In alternative or additional embodiments, the proximal side 114 and/or the distal side 116 may be non-planar. In an embodiment, the proximal side 114 and the distal side 116 may be substantially parallel to each other. Alternatively, at least a portion of the proximal side 114 and at least a portion of the distal side 116 may have a non-parallel orientation relative to each other.

The proximal side 114 is closer to the nozzles 106 than the distal side 116. The openings 104 may extend from and between the proximal side 114 and the distal side 116. One, some, or each of the openings 104 may include an inlet 118 on the proximal side 114 and an outlet 120 on the distal side 116. In other words, the fuel and/or fuel-oxidant mixture can enter the openings 104 at the inlets 118 thereof and exit at the outlets 120 thereof.

In some embodiments, the openings 104 may be configured to facilitate mixing of the fuel/oxidant therein (e.g., as the fuel/oxidant passes through the openings 104). As such, the openings 104 may improve combustion fuel/oxidant and/or stability of the flame produce during or after combustion. For example, one, some, or all of the openings 104 may be configured to initially decrease the velocity of the fuel/oxidant flowing through a portion of the openings 104. Decreasing the velocity of the fuel/oxidant may increase the time the fuel/oxidant has to mix and may create turbulent flow. In an embodiment, one, some, or all of the openings 104 may include a bulging region 122 downstream from the inlet 118, which has a larger area than the inlet 118 thereby decreasing the velocity of the fuel/oxidant.

Furthermore, in some embodiments, the openings 104 may include a sudden and/or discontinuous transition or increase in the area (unless otherwise stated, any of the areas disclosed herein are in plan view), along the downstream direction (e.g., between the inlet 118 and the bulging region 122). Hence, when the fuel/oxidant flows downstream and through the openings 104, the nominal velocity of the fuel/oxidant may at least initially decrease in the downstream direction when the area of the opening 104 increases. A nominal decrease in flow velocity in the downstream direction may be accompanied by vortex formation that can recirculate reactants and heat within the openings 104. For instance, a portion of one, some, or each of the openings 104 may be generally stepped from an intermediate location (e.g., from the bulging region 122) of the opening 104 toward and/or to the inlet 118 thereof (e.g., such that the opening 104 is wider at the bulging region 122 and may discontinuously narrow toward and/or to the inlet 118). For example, one or more of the openings 104 may include one or more steps between the bulging region 122 and the inlet 118.

As such, in some embodiments, one, some, or all of the openings 104 may include the bulging region 122 that may be located downstream from the inlet 118. In an embodiment, the bulging region 122 may be located at and/or near the outlet 120. For example, the bulging region 122 may extend from the outlet 120 to an intermediate location between the inlet 118 and the outlet 120. As such, the bulging region 122 may be located at and/or near the distal side 116 of the refractory plate 115.

The bulging region 122 may be defined by or have a maximum area A_(B) (as measured transversely to length L of the opening 104), which may be greater than an area A_(I) of the inlet 118 that is also measured transversely to the length L. In the illustrated embodiment, the maximum area A_(B) may be substantially the same as the area A_(O) of the outlet 120 that is also measured transversely to the length L.

Generally, the area A_(I) of the inlet 118, the area A_(O) of the outlet 120, the area A_(B) at or near the bulging region 122, the length L, the cross-sectional shape of the opening(s) 104, or combinations thereof may vary from one of the openings 104 to another as well as from one embodiment to another. As described above, the inlet 118 and the outlet 120 may have respective areas A_(I) and A_(O). The area of at least one of A_(I) or the area A_(O) may be in one or more of the following ranges: about 0.002 in² to about 0.01 in²; about 0.008 in² to about 0.014 in²; about 0.012 in² to about 0.019 in²; about 0.018 in to about 0.25 in²; or about 0.024 in² to about 0.040 in². It should be appreciated, however, that the areas of at least one of A_(I) or A_(O) may be less than 0.002 in² or more than 0.040 in². In some embodiments, the area A_(I) and the area A_(O) may be selected at least partially based on the expected fuel/oxidant flow rate or volume, the expected flame propagation rate (e.g., the velocity at which the fuel/oxidant burns), the amount of heat to be absorbed by the refractory plate 115, the percent of fuel/oxidant to be ignited within the flame holder 102, or combinations thereof.

The bulging region 122 may have an area A_(B) that may be greater than the area A_(I) and/or area A_(O). For example, the area A_(B) may be greater than the areas A_(I) and/or A_(O) by a percentage in one or more of the following ranges: about 5% to about 15%; about 10% to about 25%; about 20% to about 60%; about 50% to about 100%; about 80% to about 200%; about 150% to about 300%; about 250% to about 500%; or about 400% to 800%. In some embodiments, the area A_(B) may be greater than any of the areas A_(I) or A_(O) by less than 5% or by more than 800%. In another embodiment, the area A_(B) may be substantially the same as A_(O). The area A_(B) may be selected at least partially based on: the expected fuel/oxidant flow rate, the type of fuel/oxidant, the desired amount of mixing between the fuel and oxidant, the amount of heat to be absorbed by the refractory plate 115, the percent of fuel/oxidant to be ignited within the refractory plate 115, or combinations of the foregoing.

The areas A_(I) and A_(O) at one, some, or each of the openings 104 may be different from one another. In an embodiment, areas A_(I) or the area A_(O) (for the same opening 104 and/or at different openings 104) may be different from each other by a percentage in one or more of the following ranges: about 1% to about 5%; about 8% to about 15%; about 10% to about 25%; about 20% to about 50%; about 45% to about 75%; about 70% to about 150%; or about 100% to about 200%. In some embodiments, the difference between the areas A_(I) and A_(O) may be less than 1% or greater than 200% (e.g., about 200% to about 500% or about 400% to about 800%). In some embodiments, the areas A_(I) and A_(B) may be different from each other by a percentage that is less than 1% or greater than 800%.

As described above, the openings 104 may also have the length L. The length L may be the same as the thickness of the refractory plate 115. In an embodiment, the length L may also be selected at least partially based on the largest dimension of the inlet 118 and/or of the outlet 120 (e.g., based diameter of the inlet 118 and/or diameter of the outlet 120). For example, the length L may be greater than the largest dimension of the inlet 118 and/or of the outlet 120 by a percentage of about 350% to about 2000%. The length L may be selected at least partially based on the flow of fuel/oxidant, the amount of heat to be absorbed by the refractory plate 115, the amount of fuel to be ignited within the flame holder, the area A_(B), combinations of the foregoing, etc.

As previously discussed, the flame holder 102 may be partially or entirely formed from the refractory plate 115. In an embodiment, the refractory plate 115 may be monolithic and formed from a single piece. In an embodiment, the refractory plate 115 may include two or more portions stacked (e.g., positioned, abutted, connected, attached, coupled, etc.) together that collectively form the refractory plate 115. In the illustrated embodiment, the refractory plate 115 may include a first portion 128 positioned downstream from a second portion 129, which collectively form the refractory plate 115. For example, the first portion 128 may abut and/or may be attached to the second portion 129.

In an embodiment, the first portion 128 may include a first proximal side 132 spaced upstream from a first distal side 134 (e.g., distal side 116). The first proximal side 132 may include a plurality of first entrances 136 formed therein and the first distal side 136 may include a plurality of first exits 138 (e.g., outlets 120) formed therein. The areas of the first entrances 136 and the first exits 138 may be substantially the same or may be different. A portion of the openings 104 may extend through the first portion 128. For example, the first portion 128 may define a plurality of first perforations 140 extending between the first entrances 136 and the first exits 138. In the illustrated embodiment, at least one of the first perforations 140 may exhibit a substantially constant area between the first entrances 136 and the first exits 138. In another embodiment, at least some of the first perforations 140 may exhibit an area that varies (e.g., increases and/or decreases) between the first entrances 136 and the first exits 138. For example, the first perforation 140 may exhibit an area that continuously (e.g., tapered) or discontinuously (e.g., stepped) increases and/or decreases. In the illustrated embodiment, the bulging region 122 may be located within the first perforation 140.

In the illustrated embodiment, the second portion 129 may be configured substantially similar to the first portion 128. For example, the second portion 129 may include a second proximal side 142 (e.g., the proximal side 114) that is spaced upstream from a second distal side 144. The second distal side 144 may contact the first proximal side 132 to form an interface therebetween. The second portion 129 may include a plurality of second entrances 146 (e.g., inlets 118) formed in the second proximal side 142 and a plurality of second exits 148 formed in the second distal side 144. At least a portion of the openings 104 may extend through the second portion 129. For example, the second portion 129 may define a plurality of second perforations 149 extending between the second entrances 146 and the second exits 148. The second perforations 149 may exhibit a substantially constant or varied area between the second entrances 146 and the second exits 148. In other embodiments, the second portion 129 may not include the second perforation 149. Instead, for example, the second portion 129 may include a plank (FIG. 2D).

In the illustrated embodiment, the second exits 148 may exhibit an area and/or shape (in plan view) that is different (e.g., smaller) than the first entrances 136. For example, the area of the openings 104 may suddenly and/or discontinuously vary when the second exits 148 exhibit an area and/or shape that is different than the first entrances 136. In another embodiment, the second exits 148 may exhibit an area and shape (in plan view) that is substantially the same as the first entrance 136. For example, the area of the openings 104 may smoothly and/or continuously vary when the second exits 148 exhibit an area and/or shape that is substantially the same as the first entrances 136.

Referring to FIG. 1B, in the illustrated embodiment, the streamwise velocity of the fuel/oxidant is highest where the area of the opening 104 is smallest. In the illustrated embodiment, the area of the opening 104 is smallest within the second perforation 149 (e.g., at and/or near the inlet 118). For example, the velocity of the fuel/oxidant within the second perforation 149 may be greater than or equal to the rate of flame propagation to prevent the flame from leaving the refractory plate 115 and entering a space between the flame holder 102 and the nozzles 106 (e.g., flashback).

The velocity of the fuel/oxidant may decrease when the area of the opening 104 increases. In the illustrated embodiment, the area of the openings 104 increases suddenly from the second exit 148 to the first entrance 136. As such, the velocity of the fuel/oxidant may decrease when the fuel/oxidant enters the first perforation 140. In an embodiment, the velocity of the fuel/oxidant in the first perforation 140 may be less than the flame propagation rate of the fuel/oxidant. In such an embodiment, combustion of the fuel/oxidant may be trapped in the first perforation 140 so long as the velocity of the fuel/oxidant in the first perforation 140 is less than the flame propagation rate. As such, the fuel/oxidant may substantially maintain combustion of at least some of the fuel/oxidant within the first perforation 140. For instance, about 20% to substantially all of the fuel/oxidant may ignite within the first perforation 140. Additionally, the lower fuel/oxidant flow rate within one or more of the openings 104 may improve fuel oxidant mixing (e.g., as compared with openings of substantially constant area), increase the amount of fuel ignited in the openings 104, decrease the amount of NO_(x) emissions, or combinations thereof.

FIG. 1B illustrates that the sudden increase in the area of the opening 104 from the second exit 148 to the first entrance 136 may create eddies or vortices in the first perforation 140. For example, the eddies or vortices may form throughout the first perforation 140. The eddies decrease the streamwise velocity and cause heat recycling into the incoming cold fuel/oxidant. As such, the eddies may improve the flame stability of the fuel/oxidant combusted therein.

In an embodiment, the portion of the refractory plate 115 that includes the bulging region 122 may be thicker than the portion of the refractory plate 115 that does not include the bulging region 122. Hence, the fuel/oxidant is more likely to ignite in, at, and/or near the bulging region 122. For example, the first portion 128 may be thicker than the second portion 129 (e.g., about 10% to about 500% thicker or at least about 500% thicker). In another embodiment, the first portion 128 may be thinner than the second portion 129 (e.g., about 10% to about 500% thinner or at least about 500% thinner). Alternatively, the first and second portion 128, 129 may have approximately the same thickness.

It should be appreciated, however, that the first and/or second portions 128, 129 may have any suitable thickness, which may vary from one embodiment to another. In an embodiment, the first and second portions 128, 129 may have respective thicknesses of about 2 inches (5 centimeters) and about 0.5 inches (1.3 centimeters). In another embodiment, the first and second portions 128, 129 may have respective thicknesses of about 6 inches (15 centimeters) and about 1 inch (2.5 centimeters). In another embodiment, the first and second portions 128, 129 may both have thicknesses of about 2 inches (5 centimeters).

The flame holder 102 (e.g., the refractory plate 115) may be formed in any suitable manner. In an embodiment, the flame holder 102 may be formed from two or more portions stacked together (e.g. the first and second portions 128, 129) that are connected and/or joined together. For example, the first and second portions 128, 129 may be fastened, welded, or otherwise secured together. Alternatively or additionally, a portion of or the entire flame holder 102 may be substantially monolithic or formed from three or more portions. In an embodiment, at least a portion of the flame holder 102 (e.g., each of the two or more portions) may be manufactured from a solid plate by machining the openings 104 therein, such as by at least one of wire electro-discharge machining, water jet drilling, laser drilling, mechanically drilling, or any other suitable technique. In another embodiment, the first portion 128 may be supported by the second portion 129 by the force of gravity. In another embodiment, the first and second portions 128, 129 may be clamped together. In another embodiment, the first and second portions 128, 129 may be fastened together by refractory cement.

One or more components of the combustion system 100 (e.g., the flame holder 102) may be formed from any number of suitable high-temperature resistant materials or various combinations thereof. In an embodiment, the flame holder 102 may include refractory metals (e.g., niobium, molybdenum, tantalum, tungsten, rhenium, alloys thereof, or combinations thereof). In an embodiment, the flame holder 102 may include alumina silicate, cordierite, other suitable high-temperature resistant ceramics, or combinations thereof. For example, the flame holder 102 may comprise high-temperature resistant ceramic fibers, such as alumina silicate fibers. In an embodiment, the flame holder 102 may comprise any metallic material or non-metallic material exhibiting a melting temperature greater than an expected operating temperature of the flame holder 102. For example, the expected operating temperature of the flame holder 102 may be about 1600° C. to about 3500° C. In some instances, the expected operating temperature may be about 2000° C. to about 3000° C., or about 2800° C. In one or more embodiments, the flame holder 102 may include an electrically conducting material, an electrically insulating material, or combinations thereof. In an embodiment, the flame holder 102 may include a combination of any of the materials discussed herein.

In an embodiment, the flame holder 102 (e.g., the refractory plate 115) may be heated at one, some, or all of the openings 104 during combustion of the fuel/oxidant in openings 104. For example, the flame holder 102 may absorb heat by combusting the fuel/oxidant therein. The bulging region 122 may improve the heat absorption of the flame holder 102 (as compared with openings having approximately uniform areas). In another example, as will be discussed in more detail later, the flame holder 102 may be heated another device, such as an electric heating element. In an embodiment, heating the flame holder 102 may provide improved ignition of a fuel/oxidant therein (e.g., as compared with an unheated flame holder and/or with a flame holder with openings having approximately uniform areas along lengths thereof). For example, the heated flame holder 102 may be heat such that a surface of the flame holder 102 (e.g., at least one inner surface 126 defining the openings 104) may exhibit a surface temperature greater than a threshold temperature required to ignite the fuel/oxidant (e.g., an autoignition temperature). The flame holder 102 may be configured to ignite the fuel/oxidant and, thereby, may facilitate the use of a leaner fuel/oxidant and/or may not require an ignition source, such as a flame or a spark. Additionally, the heated flame holder 102 may increase a percentage of fuel/oxidant ignited in the openings 104 (as compared with an unheated flame holder and/or with a flame holder with approximately uniform openings).

For example, the average velocity of fuel/oxidant flow through the openings 104 that include the bulging region 122 may be characterized by increased vorticity compared to openings having substantially uniform area throughout.

In an embodiment, the flame holder 102 may be preheated to a temperature above the threshold temperature before the fuel is dispensed from the nozzles 106. For example, the flame holder 102 may be preheated using a flame. In another embodiment, the flame holder 102 may be preheated using an electric heating element, by an electric current passing through the flame holder 102, a laser irradiating the flame holder 102, combinations thereof, etc.

The percentage of fuel/oxidant ignited in the openings 104 may be about 20% to substantially all of the fuel. In an embodiment, greater than 50% of the fuel/oxidant that enters the inlets 118 may ignite before exiting the outlets 120 of the openings 104. In another embodiment, more that 75% of the fuel/oxidant may ignite in the openings 104. Under some conditions, a significant portion of uncombusted fuel/oxidant may ignite beyond the distal side 116 of the flame holder 102. According to embodiments, the openings 104 having a bulging region may have an increased portion of the combustion reaction occurring in the openings 104 compared to straight openings.

The heat absorbed by the flame holder 102 may be transferred to a selected location. For example, heat absorbed by the flame holder 102 may be radiated to a furnace wall or may be used to warm the fuel before the fuel is ejected by the nozzle 106 (e.g., fuel oil).

The openings 104 of the flame holder 102 may define a void fraction of the flame holder 102 (e.g., the refractory plate 115) of about 0.1 to about 0.9. Void fraction is a ratio of volume of the openings 104 to total volume of flame holder 102. In an embodiment, the flame holder 102 may have a void fraction in one or more of the following ranges: about 0.3 to about 0.8; about 0.5 to about 0.7; or about 0.60 to about 0.9. The void fraction of the flame holder may be selected at least partially based on the type of fuel, the desired temperature of the refractory plate 115, the percentage of fuel targeted for ignition within the flame holder, the fuel/oxidant flow rate, combinations of the foregoing, etc.

The nozzles 106 may be configured to dispense fuel away from an orifice thereof towards the proximal side 114. The nozzles 106 may be configured to dispense a hydrocarbon gas, such as natural gas (mostly CH₄) or propane, or hydrocarbon liquids such as fuel oil, diesel oil, etc. Additionally or alternatively, the nozzles 106 may be configured to dispense other fuels such as hydrogen or mixtures of gaseous fuels such as methane, carbon monoxide, and hydrogen. The nozzles 106 may additional dispense an oxidant and/or an additive that assists combustion. The nozzles 106 dispense the fuel at an angle and at a specific speed. For example, the nozzles 106 may dispense the fuel at about a 15° angle as measured from opposing edges of the fuel stream.

In some embodiments, the nozzles 106 may dispense the fuel at a sufficiently high velocity to prevent the flame from leaving one or more of the openings 104 and entering the space between the flame holder 102 and the nozzles 106. In an embodiment, the nozzles 106 may dispense the fuel at a velocity greater than the flame propagation velocity. Additionally, the nozzles 106 may dispense the fuel at a sufficiently low rate, such that the fuel may sufficiently mix with the oxidant prior to combustion. The combustion system 100 may be formed to be aligned with a fuel dispensed from a single nozzle 106. Alternatively, in at least one embodiment, the flame holder 102 of the combustion system 100 may be formed to be aligned with fuel dispensed from a plurality of nozzles 106.

The flame at or near the upper surface 113 may heat the flame holder 102. However, the flame at the upper surface 113 of the burner tile 110 may prevent adequate mixing of the fuel and the oxidant. Extinguishing the primary flame may facilitate movement of the flame from at or near the upper surface 113 to the flame holder 102. Systems including primary and secondary nozzles that may be used in combination with any of the embodiments disclosed herein are disclosed in U.S. Provisional Patent Application No. 61/765,022 entitled “Perforated Flame Holder and Burner Including a Perforated Flame Holder” filed on Feb. 14, 2013, the entire content of which is incorporated herein by this reference.

In some embodiments, the combustion system 100 includes a premixing region between the proximal side 114 and the nozzles 106 that allows for mixing of the fuel and an oxidant source. While the premixing region may extend from the nozzles 106 to the proximal side 114 of the flame holder 102, it will be understood that this is an approximation made for ease of understanding. Under some operating conditions, a flame may occasionally and/or briefly extend downward from the proximal side 114 of the flame holder 102. For instance, eddies in the premixing region may be temporarily bounded by a flame front and premixing may temporarily stop. However, such flame extensions may be transient, and on a time-averaged basis, the premixing region may still be considered to support premixing of the secondary fuel stream with air or flue gas. The premixing region may have a length of about 1 inch (2.54 centimeters) to about 24 inches (61 centimeters) (e.g., about 3 inches to about 8 inches (about 7.62 centimeters to about 20.32 centimeters), about 8 inches to about 16 inches (about 20.32 centimeters to about 40.64 centimeters), about 16 inches to about 24 inches (about 40.64 centimeters to about 60.96 centimeters)). The length of the premixing region may depend on the diameter of the fuel nozzle orifice, etc.

As described above, the combustion system 100 may include one or more flame holder supports 108, which may be configured to support the flame holder 102 in a furnace, boiler, or other combustion volume aligned to receive the fuel stream. The flame holder supports 108 may be configured to support the flame holder 102 substantially completely around the periphery or perimeter of the flame holder 102. The flame holder supports 108 may be fabricated from steel, from the same materials as the flame holder 102, from refractory brick, etc. For example, the flame holder supports 108 may include or be formed from a metallic superalloy, such as Inconel. In an embodiment, the flame holder supports 108 may support the flame holder 102 such that the proximal side 114 may be substantially perpendicular to the expected fuel/oxidant stream. In alternative or additional embodiments, the flame holder 102 may be supported at an angle such that the proximal side 114 may be not substantially perpendicular to the expect fuel/oxidant stream.

The flame holder supports 108 may be designed to support the flame holder 102 at a predetermined distance above the nozzles 106. In one or more embodiments, the distance between the proximal side 114 and the nozzles 106 may be about 0.5 inches to about 48 inches (about 1.27 centimeters to about 121.92 centimeters). For example, the distance between the proximal side 114 and the nozzles 106 may be about 8 inches to about 16 inches (about 20.32 centimeters to about 40.64 centimeters), or about 16 inches to about 24 inches (about 40.64 centimeters to about 60.96 centimeters). In at least one embodiment, an orifice of the one or more nozzles 106 may have a maximum orifice width and the distance between the proximal side 114 and the nozzles 106 that may be about 20 times to about 250 times greater than the maximum fuel nozzle orifice width (e.g., about 20 time to about 100 times, about 100 time to about 245 times).

The combustion system 100 may include additional equipment not shown in the illustrated embodiment. For example, the combustion system 100 may include one or more control valves that are capable of controlling the fuel and/or oxidant flow to the nozzles 106. In an embodiment, the control valves may include at least one of a manually actuated valve, a hydraulically actuated valve, or a pneumatically actuated valve. FIGS. 2A-2D are partial cross-sectional views of flame holders having at least one opening therein, according to different embodiments. Except as otherwise described herein, the flame holders shown in FIGS. 2A-2D and their materials, components, or elements may be similar to or the same as the flame holder 102 (FIGS. 1A-1B) and its respective materials, components, or elements. Any of the openings illustrated in FIGS. 2A-2D may be used in any of the flame holder embodiments disclosed herein. It is noted that while FIGS. 2A-2D illustrate only a single opening, the illustrated flame holders may include a plurality of openings formed therein arranged according to any suitable pattern including any of the patterns disclosed herein.

FIG. 2A illustrates a flame holder 202 a that is formed from a single portion (e.g., monolithic). Alternatively, the flame holder 202 a may be formed from a plurality of portions that are stacked or joined together. The flame holder 202 a includes one or more openings 204 a that extends from an inlet 218 a to an outlet 220 a. The area of the outlet 220 a is greater than the area of the inlet 218 a. The opening 204 a is defined by at least one inner surface 226 a that extends from the inlet 218 a to the outlet 220 a. The at least one inner surface 226 a exhibits a suitable convex curvature, such as a generally convex curvature, such as a parabolic, ellipsoidal, or circular curvature. As such, the opening 204 a exhibits a generally funnel-shaped opening in which an area of the opening 204 a generally smoothly and continuously increases from the inlet 218 a to the outlet 220 a (e.g., a bulging region 222 a of the opening 204 a is at and/or near the outlet 220 a). In another embodiment, the at least one inner surface 226 a may exhibit a concave curvature (e.g., the bulging region 422 c of FIG. 4C).

The shape of the opening 204 a may increase the stability and robustness of a flame combusted therein. For example, the velocity of a fuel/oxidant flowing through the opening 204 a may be greatest at and/or near the inlet 218 a and may generally decrease with distance from the inlet 218 a. The relatively high velocity of the fuel/oxidant at and/or near the inlet 218 a may prevent flashback. Additionally, the shape of the opening 204 a may facilitate combustion of the flame therein at a range of fuel/oxidant velocities and/or the velocity of the fuel/oxidant may be used to regulate where the fuel/oxidant combusts within the opening 204 a, which increases an operational stability window for a combustion system incorporating the flame holder 202 a. For example, combustion of the fuel/oxidant may occur when the velocity of the fuel/oxidant is about equal to the flame propagation rate. As such, combustion of the fuel/oxidant may move closer to the inlet 218 a when the velocity of the fuel/oxidant decreases and may move closer to the outlet 220 a when the velocity of the fuel/oxidant increases.

FIG. 2B illustrates a flame holder 202 b. For example, the flame holder 202 b includes one or more openings 204 b that extends from an inlet 218 b to an outlet 220 a. The opening 204 b is defined by at least one inner surface 226 b that is substantially planar and tapers from the outlet 220 b to the inlet 218 b. Therefore, the opening 204 b exhibits a generally truncated conical shape in which an area of the opening 204 b generally smoothly and continuously increases from the inlet 218 b to the outlet 220 b.

The flame holder 202 b may improve flame stability and robustness in substantially the same manner as the flame holder 202 a of FIG. 2A. For example, combustion of the fuel/oxidant may move closer to the inlet 218 b when the velocity of the fuel/oxidant decreases and may combust closer to the outlet 220 b when the velocity of the fuel/oxidant increases.

FIG. 2C illustrates a flame holder 202 c that is formed from a plurality of portions stacked together. For example, in the illustrated embodiment, the flame holder 202 c includes a proximal side 214 c; a distal side 216 c; and a first, second, third, fourth, and fifth portions 228 c, 229 c, 250, 252, and 254, respectively. However, the flame holder 202 c may include less or more than five portions and/or may be monolithic.

Similar to the flame holder 102, each of the plurality of portions defines an entrance and an exit. Each of the plurality of portions defines a perforation extending from the entrance to the exit. The perforations collectively form an opening 204 c that extends from an inlet 218 c to an outlet 220 c. In the illustrated embodiment, each of the perforations exhibits a different area and/or shape relative to each other. As such, the area of the opening 204 c suddenly and/or discontinuously increases from the inlet 218 c to the outlet 220 c (e.g., includes a plurality of steps) when the plurality of portions are stacked together. In another embodiment, the perforations may be configured to form an opening exhibiting an area that smoothly and/or substantially continuously increases from the inlet 218 c to the outlet 220 c.

The flame holder 202 c may improve flame stability and robustness in substantially the same manner as the flame holder 202 a of FIG. 2A. For example, combustion of the fuel/oxidant may move closer to the inlet 218 c when the velocity of the fuel/oxidant decreases and may combust closer to the outlet 220 c when the velocity of the fuel/oxidant increases. Additionally, similar to flame holder 102 of FIGS. 1A-1B, the sudden increase in the area of the opening 204 c may create eddies or vortices when the fuel/oxidant flows through the opening 204 c. For example, the eddies may facilitate combustion of fuel/oxidant over a range of fuel/oxidant velocities, which increases the operational stability window for a combustion system incorporating the flame holder 202 c.

FIG. 2D illustrates a flame holder 202 e that is formed from a plurality of portions. For example, the flame holder 202 e includes a first portion 228 e that is downstream from a second portion 229 e. The first portion 228 e defines an entrance 236 e, an exit 238 e downstream from the entrance 236 e, and a perforation 240 e extending therebetween. The entrance 236 e may exhibit a maximum width W_(F). The second portion 229 e may exhibit a plank-like structure having a length (e.g., measured traverse to the maximum width W_(F)) sufficient to span across the entrance 236 e. The second portion 229 e may also exhibit a maximum width W_(S) that is measured is a direction substantially parallel to the maximum width W_(F). The maximum width W_(F) of the perforation 240 e is greater than the maximum width W_(S) of the second portion 229 e. As such, the second portion 229 e cannot completely obscure the entrance 236 e.

The second portion 229 e may be stacked relative to the first portion 228 e to completely span across the perforation 240 e and only partially obstruct the entrance 236 e. For example, in the illustrated embodiment, the second portion 229 e may obstruct a middle section of the entrance 236 e. As such, the entrance 236 e includes a first section 260 and a second section 262 located on either side of the second portion 229 e that are not obstructed by the second portion 229 e. As such, the first and second section 260, 262 may collectively form an inlet 218 e having an area that is less than the area of the entrance 236 e. In another embodiment, the second portion 229 e may only obstruct a side section of the entrance 236 e. As such, the entrance 236 e only includes a single section that is located on one side the second portion 229 e. In another embodiment, the second portion 299 e may be staked relative to the first portion 228 e to only partially span across the entrance 236 e. In another embodiment, the second portion 229 e may not exhibit a length sufficient to span across the entrance 236 e. As such, the second portion 229 e may only span across a portion of the entrance 236 e or be supported in the middle of the entrance 236 e (e.g., using wires made out of any of the high-temperature resistant materials disclosed herein). In another embodiment, the entrance 236 e may be partially obstructed by a plurality of second portions 229 e.

In operation, the flame holder 202 e may operate substantially similar to the flame holder 102 a (FIGS. 1A-1B). For example, the sudden increase in the area of the opening 204 e may create eddies or vortices when the fuel/oxidant flows through the opening 204 e.

FIG. 3 is an isometric cutaway view of a combustion system 300, according to an embodiment. Except as otherwise described herein, the combustion system 300 and its materials, components, or elements may be similar to or the same as the combustion system 100 (FIGS. 1A-1B) and its respective materials, components, or elements. For instance, the combustion system 300 may include one or more nozzles 106 spaced from and oriented toward a flame holder 302. The nozzles 106 may dispense a fuel/oxidant, which may generally flow toward the flame holder 302 and/or at least some of the fuel/oxidant may flow through a plurality of openings 304 defined by the flame holder 302.

The flame holder 302 includes a proximal side 314 and/or a distal side 316. The proximal side 314 may be closer to the one or more nozzles 106 than the distal side 316. The proximal side 314 and the distal side 316 may define a plurality of inlets 318 and a plurality of outlets 320 therein, respectively. One, some, or each of the plurality of openings 304 may extend from an inlet 318 to an outlet 320. The fuel/oxidant emitted from the one or more nozzles 106 may enter the openings 304 at the inlets 318 and exit at the outlets 320 thereof. In the illustrated embodiment, the flame holder 302 is collectively formed from two or more portions (e.g., a first portion 328 and a second portion 329 upstream from the first portion 328). In another embodiment, the flame holder 302 may be monolithic.

In some embodiments, the openings 304 may be configured to facilitate mixing and combustion of a fuel/oxidant flowing therein. As such, the openings 304 may improve the stability and robustness of the flame produced during or after combustion. For example, one, some, or all of the openings 304 may be configured to initially reduce the velocity of the fuel/oxidant flowing through a portion of the openings 304 using any of the methods disclosed herein. For example, one, some, or all of the openings 304 may exhibit a bulging region 322 that is positioned downstream from the inlet 318, which has a larger area than the inlet 318.

Furthermore, in some embodiments, the openings 304 may include a gradual or substantially continuous transition or increase of the cross-section area along the downstream direction, between the inlet 318 and an intermediate location (e.g., the bulging region 322) of the opening 304. For example, a portion of one, some, or each of the openings 304 may be generally tapered from the bulging region 322 toward and/or to the inlet 318 thereof (e.g., such that the opening 304 is wider at the bulging region 322 and may substantially continuously narrows toward and/or to the inlet 318). In other embodiments, as will be discussed in more detail below, the openings 304 may include a sudden and/or discontinuous transition or increase of the area along the downstream direction (e.g., stepped).

In additional or alternative embodiments, at least a portion of one, some, or each of the openings 304 may be configured to facilitate velocity increase of the fuel/oxidant flow in the downstream direction through another portion of the openings 304 (e.g., from the bulging region 322 toward and/or to the outlet 320 of the opening). For example, the area of the opening 304 at the bulging region 322 may be greater than the area of the outlet 320. In an embodiment, the area of the opening 304 may gradually or substantially continuously decrease from the bulging region 322 toward and/or to the outlet 320 (e.g., tapered). In another embodiment, the area of the opening 304 may suddenly and/or discontinuously decrease from the bulging region 322 toward and/or to the outlet 320. (e.g., stepped).

In at least one embodiment, at least a portion of one, some, or all of the openings 304 may have a generally circular shape in plan view. Hence, for example, one or more portions of one, some, or each of the openings 304 may have approximately conical shapes (e.g., a shape of a truncated cone). In an embodiment, one, some, or each of the openings 304 may have two conical portions that may have bases thereof connected to each other and forming the bulging region 322 of the opening 304. As described in more detail below, the openings 304 may have any number of suitable configurations and shapes, which may vary from one embodiment to the next.

In some embodiments, one, some, or all of the openings 304 may include a bulging region 322 that is spaced from the inlet 318 and the outlet 320. The bulging region 322 may be defined by or have a maximum area A_(B) (as measured transversely to length L of the opening 304), which may be greater than area Ai of the inlet 318 and area A_(O) of the outlet 320.

In an embodiment, the bulging region 322 may be located midway between the proximal and distal sides 314, 316 of the flame holder 302. Hence, the maximum area A_(B) may be located approximately at the centerline between the proximal side 314 and the distal side 316. In another embodiment, the bulging region 322 may be located more proximate the distal side 316 than the proximal side 314 or more proximate the proximal side 314 than the distal side 316.

As previously discussed, the openings 304 illustrated in FIG. 3 may improve flame stability and robustness. For example, the initial velocity of the fuel/oxidant may be relatively high at and/or near the inlets 318. The relatively high velocity of the fuel/oxidant may be greater than the flame propagation rate, thereby substantially preventing flashback. The velocity of the fuel/oxidant may decrease from a location at and/or near the inlets 318 toward and/or to the bulging regions 322 and/or increase from the bulging regions 322 toward and/or to the outlets 320. Decreasing and/or increasing the velocity of the fuel/oxidant may improve mixing thereof, regulate where the fuel/oxidant combusts in the openings 304 (e.g., combustion of the fuel/oxidant moves towards the outlet 320 when the velocity of the fuel/oxidant increases), improve combustion of a fuel/oxidant having relatively high velocities, improve combustion of a leaner fuel/oxidant, better trap combustion of the fuel/oxidant within the openings 304 over a range of fuelioxidant velocities compared to an opening having a constant area, or combinations thereof. In another embodiment, the relatively small area A_(I) and A_(O) of the inlets 318 and outlets 320, respectively, relative to the area A_(B) of the bulging region 322, may increase the amount of heat absorbed by flame holder 302. The increased about of heat absorbed by the flame holder 302 may increase the temperature of at least one surface of the flame holder 302 (e.g., at least one inner surface 326 defining the opening 304) above an autoignition temperature of the fuel/oxidant which may, in turn, increase the amount of the fuel/oxidant that combusts in the opening 304.

The location within the opening 304 where the fuel/oxidant combusts may be controlled depending on the velocity of the fuel/oxidant. Similar to the openings 204 a, 204 b, and 204 c (FIGS. 2A-2C), the fuel/oxidant may combust at a location within the opening 304 that is more proximate the outlet 320 when the velocity of the fuel/oxidant is increased and may combust at a location within the opening 304 that is more proximate the inlet 318 when the velocity of the fuel/oxidant is decreased.

FIGS. 4A-4F are partial cross-sectional views of flame holders, according to different embodiments. Except as otherwise described herein, the flame holders shown in FIGS. 4A-4F and their respective elements and components may be similar to or the same as any of the flame holders 102, 202 a-e, 302 (FIGS. 1A-3) and their respective elements and components. Any of the openings illustrated in FIGS. 4A-4F may be used in any of the flame holder embodiments disclosed herein. It is noted that while FIGS. 4A-4F illustrate only a single opening, the illustrated flame holders may include a plurality of openings formed therein arranged according to any suitable pattern including any of the patterns disclosed herein.

FIG. 4A is a partial cross-sectional view of a flame holder 402 a that includes a first portion 428 a, a second portion 429 a upstream from the first portion 428 a, and at least one opening 404 a. The first portion 428 a may be thicker than the second portion 429 a. For instance, a bulging region 422 a may be offset from a centerline of the flame holder 402 a between the proximal side 414 a and the distal side 416 a (e.g. the bulging region 422 a may be closer to the distal side 414 a than proximal side). In an embodiment, the opening 404 a may include an inlet 418 a that has a smaller area than an outlet 420 a.

The flame holder 402 a may improve flame stability and robustness in substantially the same manner as the flame holder 302 of FIG. 3. For example, a fuel/oxidant may have a velocity at and/or near the inlet 418 a that is greater than the flame propagation rate thereby substantially preventing flashback. In another example, combustion of the fuel/oxidant may move closer to the outlet 420 a when the velocity of the fuel/oxidant increases and may move closer to the inlet 418 a when the velocity of the fuel/oxidant decreases.

In some embodiments, the bulging region may have a largest area at a point or that may lie along a line or a plane. Alternatively, the largest area of the bulging region may extend along the length of the opening. FIG. 4B is a partial cross-sectional view of a flame holder 402 b includes at least one opening 404 b, which has a bulging region 422 b that extends along a length of the opening 404 b. For instance, along the length of the opening 404 b, the bulging region 422 b may have a constant area in plan view. For example, the bulging region 422 may have a constant diameter or other lateral dimension. Additionally or alternatively, as mentioned above, in an embodiment, the flame holder 402 b may be formed from a single piece of material.

The flame holder 402 b may improve flame stability and robustness in substantially the same manner as the flame holder 302, 402 a of FIGS. 3-4A. For example, a fuel/oxidant may have a velocity at and/or near the inlet 418 a that is greater than the flame propagation rate, thereby substantially preventing flashback. The velocity of the fuel/oxidant may generally decrease from the inlet 418 b to the bulging region 422 b and generally increase from bulging region 422 b to the outlet 420 b. However, the velocity of the fuel/oxidant may be relatively constant in the bulging region 422 b due to the relatively constant area of the bulging region 422 b along a length of the opening 404 b. The relatively constant velocity of the fuel/oxidant may improve mixing of the fuel/oxidant, increase the percentage of the fuel/oxidant that combusts in the opening 404 b, improve absorption of the heat into the flame holder 402 b, etc.

Furthermore, as described above, the cross-sectional shape of the openings (e.g., at cross-section taken along the length of the opening) may vary from one embodiment to the next. FIG. 4C is a partial cross-sectional view of a flame holder 402 c that includes at least one opening 404 c, which may have at least one curved inner surface 426 c. For example, the opening 404 c may have a concave curvature extending between inlet 418 c and outlet 420 c of the opening 404 c. In an embodiment, a bulging region 422 c may be formed at location where a slope of the curved inner surface 426 c is substantially vertical. It should be appreciated that the openings in the flame holder may have any number of suitable configurations that may include a bulging region between inlets and outlets thereof.

The flame holder 402 c may improve flame stability and robustness in substantially the same manner as the flame holder 302, 402 a-b (FIGS. 3-4B). For example, a fuel/oxidant may have a velocity at and/or near the inlet 418 a that is greater than the flame propagation rate, thereby substantially preventing flashback.

Furthermore, the opening may exhibit a sudden and/or discontinuous transition, increase, and/or decrease along the length of the opening. FIG. 4D is a partial cross-sectional view of a flame holder 402 d that includes a plurality of portions that collectively form the flame holder 402 d. Each of the plurality of portions may define a perforation and the perforations may collectively define an opening 404 d. The opening 404 d may extend from an inlet 418 d to an outlet 420 d and include a bulging region 422 d therebetween. In the illustrated embodiment, the opening 404 d may exhibit a stepped shaped. For example, at least some of the perforations exhibit an area that is different than the area of another perforation. The plurality of portions may be stacked such that the areas of the perforations generally increases from the inlet 418 d to the bulging region 422 d and decrease from the bulging region 422 d to the outlet 420 d.

The flame holder 402 d may improve flame stability and robustness in substantially the same manner as the flame holder 302, 402 a-c (FIGS. 3-4C). For example, a fuel/oxidant may have a velocity at and/or near the inlet 418 d that is greater than the flame propagation rate, thereby substantially preventing flashback. Additionally, the fuel/oxidant may form eddies or vortices at and/or near where the area of the opening 404 d suddenly increases, similar to the openings 104, 204 c, 204 e (FIGS. 1B, 2C, and 2D).

FIG. 4E illustrates a flame holder 402 e that is formed from a plurality of portions. In the illustrated embodiment, the flame holder 402 e includes a first portion 428 e, a second portion 429 e, and a third portion 450 e. The first portion 428 e may include an entrance 436 e, and exit 438 e downstream from the entrance 436 e, and a perforation 440 e extending therebetween that at least partially forms an opening 404 e. The entrance 436 e exhibits a maximum width W_(EN) and the exit 438 e exhibits a maximum width W_(EX). In the illustrated embodiment, the maximum width W_(EN) and the maximum width W_(EX) are substantially the same. However, in other embodiments, the maximum width W_(EN) and W_(EX) may be different. The second and third portions 429 e, 450 e comprise a plank-like shape having a length in a longitudinal direction thereof and a width W_(S) and W_(T), respectively, measured substantially parallel to the widths W_(EN) and W_(EX). The widths W_(S) and W_(T) may be less than the maximum widths W_(EN) and W_(EX), respectively. The width W_(S) and W_(T) may be substantially the same or different.

Similar to the flame holder 202 e (FIG. 2D), the second and third portions 429 e, 450 e may partially obstruct the entrance 436 e and the exit 438 e, respectively, when the second and third portions 429 e, 450 e are stacked relative to the first portion 428 e. In the illustrated embodiment, the second and third portions 429 e, 450 e may obstruct a middle section of the entrance 436 e and exit 438 e, respectively. As such, the entrance 436 e may include a first section 460 and a second section 462 located on either side of the second portion 429 e that are unobstructed by the second portion 429 e. Similarly, the exit 438 e may include a first section 466 and a second section 468 on either side of the third portion 450 e that are unobstructed by the third portion 450 e. The first and second sections 460, 462 of the entrance 436 e may collectively form an inlet 418 e and the first and second sections 466, 468 of the exit 438 e may collectively form an outlet 420 e. In another embodiment, the second and/or the third portion 429 e, 450 e may only obstructs a side section of the entrance 436 e and/or the exit 438 e, respectively, when the second and/or third portion 429 e, 450 e are stacked relative to the first portion 428 e. In another embodiment, the entrance 436 e and/or the exit 438 e may be at least partially obstructed by a plurality of second portions 429 e and/or a plurality of third portions 450 e, respectively.

The flame holder 402 e may improve flame stability in substantially the same manner as the flame holder 302, 402 a-d (FIGS. 3-4D). For example, a fuel/oxidant may have a velocity at and/or near the inlet 418 e that is greater than the flame propagation rate, thereby substantially preventing flashback. Additionally, the fuel/oxidant may form eddies at and/or near where the area of the opening 404 e suddenly increases, similar to the openings 104, 204 c. 204 e, 404 d (FIGS. 1A-1B, 2C, 2D, and 4D).

FIG. 4F illustrates a flame holder 402 f that is collectively formed from a plurality of portions. In the illustrated embodiment, the flame holder 402 f may be collectively formed from a first portion 428 f, a second portion 429 f upstream from the first portion 428 f, and a third portion 450 f downstream from the first portion 428 f. Each of the first, second, and third portions 428 f, 429 f, and 450 f may define a first entrance 436 f, a second entrance 446, and a third entrance 470, respectively; a first exit 438 f, second exit 448, and a third exit 472, respectively; and a first perforation 440 f, a second perforation 449, and a third perforation 474, respectively. Each of the first, second, and third perforations 440 f, 449, 474 collectively form an opening 404 f. Additionally, the second entrance 446 may form an inlet 418 f of the opening 404 f and the third exit 472 may form an outlet 420 f of the opening 404 f.

In the illustrated embodiment, an area of the opening 404 f may suddenly and discontinuously increase from the inlet 418 f to an intermediate location (e.g., the first perforation 440 f, a bulging region 422 f, etc.) and may suddenly and discontinuously decrease from the intermediate location to the outlet 420 f. For example, the second exit 448 may exhibit an area and/or shape that is different (e.g., less than) the area and/or shape of the first entrance 436 f. Similarly, the first exit 438 f may exhibit an area and/or shape that is different (e.g., greater than) the area and/or shape of the third entrance 470. In another embodiment, the first, second, and third portions 428 f, 429 f, 450 f may be configured to form an opening exhibiting an area that smoothly and continuously increases from the inlet 418 f to the intermediate location and/or may smoothly and continuously decreases from the intermediate location to the outlet 420 f.

The flame holder 402 e may improve flame stability in substantially the same manner as the flame holder 302, 402 a-e (FIGS. 3-4E). For example, a fuel/oxidant may have a velocity at and/or near the inlet 418 d that is greater than the flame propagation rate, thereby substantially preventing flashback. Additionally, the fuel/oxidant may form eddies at and/or near where the area of the opening 404 f suddenly increases, similar to the openings 104, 204 c. 204 e, 404 d, 404 e (FIGS. 1A-1B, 2C, 2D, and 4D-4E).

FIG. 5 is a partial cross-sectional view of a flame holder 502 defining an opening 504, according to an embodiment. Except as otherwise described herein, the flame holder 502 shown in FIG. 5 and its respective elements and components may be similar to or the same as any of the flame holders 102, 202 a-e. 302, 402 a-f (FIGS. 1A-4F) and their respective elements and components. It is noted that while FIG. 5 illustrate only a single opening, the illustrated flame holder may include a plurality of openings formed therein arranged according to any suitable pattern including any of the patterns disclosed herein.

In the illustrated embodiment, the opening 504 extends from an inlet 518 to an outlet 520. The opening 504 exhibits an area that generally decreases from the inlet 518 to a narrowed region 576. The area of the opening 504 then increases from the narrowed region 576 to another location of the opening 504 (e.g., an intermediate location, a bulging region 522, and/or the outlet 520). The area of the opening 504 may increase and/or decrease in a smooth and continuous manner or in a sudden and discontinuous manner.

In an embodiment, decreasing the area of the opening 504 from the inlet 518 to the narrowed region 576 may improve the aerodynamics of the flame holder 502. For example, referring to FIGS. 1A-1B, the velocity of a fuel/oxidant may decrease between nozzles 106 and the flame holder 102. The decrease in the velocity of the fuel/oxidant may be at least partially caused by the non-aerodynamic shape of the flame holder 102 (e.g., relatively small areas A_(I) relative to the surface area of the proximal side 114). As such, referring to FIG. 5, increasing the size of the inlet 518 may improve the aerodynamics of the flame holder 502 and may increase the velocity of the fuel/oxidant flowing into the opening 504.

In operation, the velocity of the fuel/oxidant may increase from the inlet 518 to the narrowed region 576. The velocity of the fuel/oxidant at the narrowed region 576 may be greater than the flame propagation rate of the fuel/oxidant thereby substantially preventing flashback. Additionally, the velocity of the fuel/oxidant flowing through the narrowed region 576 may be greater than the velocity of a fuel/oxidant flowing through an opening having the same area as the narrowed region 576 (e.g., all other conditions the same) because of the improved aerodynamics of the flame holder 502. The velocity of the fuel/oxidant than then decrease from the narrowed region 576 to another region of the opening (e.g., the bulging region 522 and/or the outlet 520).

The flame holder 502 may be used in any of the flame holder embodiments disclosed herein. For example, any of the flame holders 102, 202 a-e, 302, 402 a-f (FIGS. 1A-4F) may include an opening exhibiting an area that generally decreases from the inlet to a narrowed region thereof and generally increase from the narrowed region to another region of the opening (e.g., an intermediate location, a bulging region, and/or an outlet).

FIGS. 6A and 6B are partially cross-sectional views of flame holders that include one or more mechanisms disposed in an opening thereof that increase mixing of the fuel/oxidant, according to one or more embodiments. Increasing the mixing of the fuel/oxidant may also improve flame stability and robustness by improving combustion of leaner fuels and/or poorly mixed fuel/oxidant. Except as otherwise described herein, the flame holders shown in FIGS. 6A-6B and their respective elements and components may be similar to or the same as any of the flame holders 102, 202 a-e, 302, 402 a-g, 502 (FIGS. 1A-5) and their respective elements and components. The flame holders shown in FIGS. 6A-6B may be used in any of the flame holders disclosed herein. It is noted that while FIGS. 6A-6B illustrate only a single opening, the illustrated flame holders may include a plurality of openings formed therein arranged according to any suitable pattern including any of the patterns disclosed herein.

FIG. 6A illustrates a flame holder 602 a including an opening 604 a therein. The opening 604 a is defined by at least one inner surface 626 a of the flame holder 602 a. At least a portion of the inner surface 626 a may include one or more textured features 678 formed (e.g., machined) therein. The textured features 678 may include one or more protrusions, recesses, notches, channels, fins, divots, or other suitable textured feature. The textured features 678 may form a corkscrew-like pattern, a checkered pattern, or any other suitable pattern configured to cause at least some of the fuel/oxidant flowing within the opening 604 to swirl and/or exhibit turbulent flow and generate eddies or vortices, which may increase mixing of the fuel/oxidant.

FIG. 6B illustrates a flame holder 602 b including an opening 604 b therein. The opening 604 b is defined by at least one inner 626 b of the flame holder 602 b. The opening 604 b may include one or more turbulators 680 positioned therein that are attached to the at least one inner surface 626 b or otherwise secured within the opening 604 b. The turbulators 680 may be configured to increase the turbulence of a fuel/oxidant flowing through the opening 604 b. The turbulators 680 may be formed from any of the high-temperature resistant materials disclosed herein.

In the illustrated embodiment, the turbulators 680 includes an elongated member 682 attached to the inner surface 626 b at a first end thereof. The turbulators 680 may further include a plate 684 attached to a second end of the elongated member 682. The plate 684 may exhibit an oblique angle or perpendicular angle relative to a length L of the opening 604. The plate 684 and/or the elongated member 682 may increase turbulent flow of the fuel/oxidant. The turbulent flow may increase the mixing of the fuel/oxidant, form eddies, combinations thereof, etc. In another embodiment, the turbulators 680 may include a coiled rod, a wire mesh, or any other suitable turbulator.

FIG. 7 is a partial cross-sectional view of a flame holder 702 including sleeve 786 that is attachable to a refractory plate 715 or other type of refractory body, according to an embodiment. Except as otherwise described herein, the flame holder 702 shown in FIG. 7 and its respective elements and components may be similar to or the same as any of the flame holders 102, 202 a-e, 302, 402 a-g, 502, 602 a-b (FIGS. 1A-6B) and their respective elements and components. For example, the sleeve 786 may be formed from any of the high-temperature resistant materials disclosed herein. The sleeve 786 and the refractory plate 715 shown in FIG. 7 may be used in any of the flame holders disclosed herein. It is noted that while FIG. 7 illustrates only a single opening 704 (e.g., sleeve 786), the illustrated flame holder may include a plurality of openings formed therein arranged according to any suitable pattern including any of the patterns disclosed herein.

In the illustrated embodiment, the sleeve 786 includes at least one side wall 788 having at least one inner surface 726 and at least one outer surface 790 spaced from the inner surface 726. The inner surface 726 defines an inlet 718, an outlet 720 downstream from the inlet 718, and an opening 704 extending therebetween. The opening 704 may be substantially the same as or similar to any of the openings disclosed herein. The inner surface 726 may also include one or more textures surfaces 778 formed therein and/or turbulators extending therefrom.

The refractory plate 715 may define a conduit 792. The conduit 792 may exhibit a size and shape configured to receive the sleeve 786. For example, the conduit 792 may exhibit a shape that substantially corresponds to a shape of the outer surface 790. For instance, the conduit 792 may exhibit a width We that is substantially the same as or slightly greater than a maximum width W_(O) of the outer surface 790.

In an embodiment, the sleeve 786 may be attached to the refractory plate 715. For example, the sleeve 786 may be attached to the flame holder 702 by welding, press-fitting, using a high-temperature resistant adhesive, a mechanical fastener, or any other suitable technique. In another embodiment, the sleeve 786 may reversibly attached to the refractory plate 715 thereby allowing the flame holder 702 to be adapted for different applications.

FIGS. 8A-8D illustrate top plan views of a distal side of flame holders 802 a, 802 b, 802 c, 802 d, according to one or more embodiments. Except as otherwise described herein, any of the flame holders 802 a. 802 b, 802 c, 802 d as well as elements and components thereof may be similar to or the same as the flame holders 102, 202 a-e, 302, 402 a-g, 502, 602 a-b, 702 (FIGS. 1A-7) and its corresponding elements and components. For example, as shown in FIG. 8A, the flame holder 802 a may include openings 804 a, 804 a′, 804 a″, which may be similar to or the same as one or more of the openings 104, 204 a-e, 304, 404 a-g, 504, 604 a-b, 704 (FIGS. 1A-7). Additionally or alternatively, the openings 804 a, 804 a′, 804 a″ may include corresponding bulging regions 822 a. 822 a′, 822 a″ (shown as phantom lines) at intermediate locations between the inlets and outlets thereof.

Referring to FIG. 8A, in some embodiments, the opening 804 a may be positioned approximately in the center of the flame holder 802 a and the openings 804 a′ and 804 a″ may be arranged circularly or concentrically about the opening 804 a. For example, openings 804 a′ may be arranged in two circular arrangements, where some of the openings 804 a′ form a first circular arrangement (e.g., centered about the opening 804 a) and other openings 804 a′ form a second circular arrangement that encloses or encircles the first circular arrangement. Additionally or alternatively, openings 804 a″ may be arranged circularly (e.g., the openings 804 a′″ may encircle at least some of the openings 804 a′).

In one or more embodiments, the openings 804 a′ and 804 a″ may have a similar or the same radial distance therebetween. The openings 804 a, 804 a′, 804 a″ have generally circular cross-sections. In an embodiment, the opening 804 a may be larger than the openings 804 a′ and/or openings 804 a″. In some embodiments, the openings 804 a′ may be larger than the openings 804 a″. In other words, in some embodiments, the flame holder 802 a may have openings of three different sizes. It should be appreciated, however, the flame holder 802 a may include any number of openings of any number of suitable sizes, which may vary from one embodiment to another (e.g., the flame holder 802 a may have openings of two, three, four, five, etc., different sizes).

Furthermore, the openings of the flame holder may have any number of suitable arrangements. FIG. 8B illustrates a top view of the flame holder 802 b with openings 804 b having a generally spiral arrangement, according to an embodiment. The openings 804 b may be arranged in a helical pattern having substantially same helical spacing between the openings 804 b. For example, the flame holder 802 b may include an opening 804 b at or near the center thereof, while additional openings 804 b that may be arranged along a substantially helical pattern that spirals outward from the opening 804 b at or near the center of the flame holder 802 b. In some embodiments, radial and/or lateral spacing between the openings 804 b may be approximately constant along the helical pattern thereof. In the illustrated embodiment, the openings 804 b may exhibit a generally circular cross-sectional shape or any other suitable shape.

As described above, the openings 804 a. 804 a′, 804 a″. 804 b may have generally circular cross-sectional shapes. It should be appreciated, however, that cross-sectional shapes of the openings may vary from one embodiment to the next. For example, FIG. 8C is a top view of the flame holder 802 c with openings 804 c that have approximately triangular shapes in plan view, according to an embodiment. In some embodiments, the openings 804 c may be arranged along equally spaced linear paths. For instance, the openings 804 c positioned along a first path may be offset from the openings 804 c arranged along another path.

Furthermore, in some embodiments, the openings 804 c may be arranged along paths that are approximately parallel one to another. For example, one or more sides of the openings 804 c positioned along the first path may be approximately parallel to one or more sides of the openings 804 c positioned along another path that is approximately parallel to the first path. In an embodiment, the one, some, or all of the openings 804 c may lie along two or more paths. For instance, one, some, or all of the openings 804 c may lie along at least two paths that have non-perpendicular orientation relative to each other.

In some embodiments, the openings 804 c may have approximately the same cross-sectional shape along the entire lengths thereof (e.g., along the thickness of the flame holder 802 c). For example, bulging regions 822 c, which may be positioned between the inlets and outlets of the openings 804 c, may have a generally triangular shape. Additionally or alternatively, in at least one embodiment, the cross-sectional shape of one, some, or all of the openings 804 c may vary along respective lengths thereof (e.g., the opening 804 c may have different cross-sectional shapes at the inlet and outlet, at the outlet and bulging region 822 c, at the inlet and bulging region 822 c, etc.).

FIG. 8D is a top view of the flame holder 802 d with openings 804 d that have inlets and/or outlets with different shapes from bulging regions 822 d thereof, according to an embodiment (e.g. in cross-sections take transversely to the lengths of the openings 804 d or to the thickness of the flame holder 802 d). In an embodiment, the inlets and outlets of the openings 804 d may have approximate rectangular or square cross-sectional shapes and the bulging regions 822 d may have approximately circular cross-sectional shapes. Also, in some embodiments, the inlets and outlets of the openings 804 d may have different cross-sectional shapes one from another. In any event, the area at the bulging regions 822 d may be larger than corresponding areas at the inlets and/or outlets of the openings 804 d.

Moreover, in some embodiments, the openings 804 d may be arranged in a grid-like or periodic pattern. For instance, the flame holder 802 d may have the same lateral spacing between the openings 804 d. In an embodiment, the openings 804 d may be arranged along approximately linear paths. For example, one, some, or all of the openings 804 d may lie on two approximately perpendicular paths.

FIG. 9 is a top plan view of a flame holder assembly 900 that includes a plurality of flame holders 902, according to an embodiment. Generally, the flame holders 902 may include any of the flame holders described herein, which may be arranged in any number of patterns. For example, the flame holders 902 may be similar to or the same as the flame holder 102 (FIGS. 1A and 1B) and may be arranged in a circular pattern around an opening 904 in a main plate 906. In particular, the main plate 906 may define one or more holes 908 therein that may accept flame holders 902, which may be secured to the main plate 906 therein.

While various aspects and embodiments have been disclosed, other aspects and embodiments may be contemplated. The various aspects and embodiments disclosed here are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

1. A flame holder, comprising: a refractory body defining a proximal side, a distal side spaced downstream from the proximal side, and a plurality of openings extending therethrough, each of the plurality of openings extending between an inlet on the proximal side and an outlet on the distal side, one or more of the plurality of openings including: a bulging region located downstream from the inlet thereof; and an area in plan view that decreases from the bulging region to the outlet.
 2. (canceled)
 3. The flame holder of claim 1 wherein the the refractory body includes: a first portion including a first proximal side and a first distal side spaced downstream from the first proximal side, the first portion defining a plurality of first perforations extending therethrough, each of the plurality of first perforations extending between a first entrance on the first proximal side and a first exit on the first distal side, each of the plurality of first perforations at least partially forming one of the plurality of openings; and a second portion including a second proximal side and a second distal side spaced downstream from the second proximal side, the second distal side stacked adjacent to the first proximal side of the first portion, the second portion at least partially obstructing the first entrance of at least one of the plurality of first perforations.
 4. The flame holder of claim 3 wherein the second portion defines a plurality of second perforations extending therethrough, each of the plurality of second perforations extending between a second entrance on the second proximal side and a second exit on the second distal side, each of the plurality of first and second perforations at least partially defining a corresponding one of the plurality of openings.
 5. The flame holder of claim 3 wherein the first entrance of at least one of the plurality of first perforations includes one or more sections thereof that are not obstructed by the second portion, the one or more sections of the first entrance of at least one of the plurality of first perforations forms the inlet.
 6. The flame holder of claim 3, wherein the refractory plate includes at least one third portion including a third proximal side and a third distal side spaced downstream from the third proximal side, the third proximal side stacked adjacent to the first distal side of the first portion, the at least one third portion at least partially obstructing the first exit of at least one of the first perforations.
 7. The flame holder of claim 1 wherein the one or more of the plurality of openings exhibits at least one of a generally tapered shape, a generally stepped shape, or one or more curved inner surfaces. 8-10. (canceled)
 11. The flame holder of claim 1 wherein at least one of the plurality of openings include one or more textured features formed on at least a portion of at least one inner surface that at least partially defines the at least one of the plurality of openings.
 12. The flame holder of claim 1 wherein the flame holder has a void fraction greater than about 0.7.
 13. The flame holder of claim 1 wherein the bulging region of at least one of the one or more of the plurality of openings has a generally uniform lateral dimension along a length thereof.
 14. The flame holder of claim 1 wherein the refractory body includes at least one of a refractory metal, alumina silicate, or cordierite.
 15. The flame holder of claim 1 wherein at least some of the plurality of openings include one or more turbulators positioned therein.
 16. The flame holder of claim 1, further comprising a plurality of sleeves, each of the plurality of sleeves including at least one wall at least partially defining at least one of the plurality of openings, wherein the refractory body defines a plurality of conduits therein, each of the plurality of conduits configured to receive one of the plurality of sleeves.
 17. A combustion system, comprising: a flame holder including: a refractory body defining a plurality of openings therein, one or more of the plurality openings including: an inlet; a bulging region located downstream from the inlet; an outlet located downstream from the inlet and the bulging region; and an area in plan view that decreases from the bulging region to the outlet; one or more nozzles configured to dispense fuel toward the flame holder and into the plurality of openings thereof; and one or more flame holder supports supporting the flame holder above the one or more nozzles to define a standoff between the one or more nozzles and the flame holder.
 18. The combustion system of claim 17 wherein the inlet of the one or more of the plurality of openings has a first area in plan view and the bulging region has a second area in plan view that is larger than the first area.
 19. (canceled)
 20. The combustion system of claim 17, further comprising a burner tile at least partially surrounding at least one of the one or more nozzles.
 21. The combustion system of claim 17 wherein the one or more nozzles include one or more primary nozzles and one or more secondary nozzles, the one or more primary nozzle being positioned and configured to ignite the fuel dispensed from the one or more secondary nozzles.
 22. A method of operating a combustion system, the method comprising: dispensing fuel from one or more nozzles toward inlets of a plurality of openings of a flame holder and further toward outlets of the plurality of openings of the flame holder, decreasing a velocity of the fuel as the fuel passes through at least a portion of the plurality of openings; igniting at least a portion of the fuel in the flame holder; and after decreasing the velocity of the fuel, increasing the velocity of the fuel as the fuel passes through the at least a portion of the plurality of openings.
 23. (canceled)
 24. The method of claim 22, further comprising mixing at least a portion of the fuel with an oxidant in the one or more openings of the flame holder.
 25. The method of claim 22, further comprising mixing at least a portion of the fuel with an oxidant in a region between the one or more nozzles and the flame holder.
 26. The method of claim 22, wherein dispensing fuel from one or more nozzles includes dispensing a mixture of the fuel with an oxidant.
 27. The method of claim 22, further comprising igniting at least a portion of the fuel in the plurality of openings.
 28. A flame holder, comprising: a refractory body defining a proximal side, a distal side spaced downstream from the proximal side, and a plurality of openings extending therethrough, each of the plurality of openings extending between an inlet on the proximal side and an outlet on the distal side, one or more of the plurality of openings including: a bulging region located downstream from the inlet thereof; and an area in plan view that generally continuously increases from the inlet to the bulging region.
 29. The flame holder of claim 28 wherein the one or more of the plurality of openings exhibits at least one of a generally tapered shape or one or more curved inner surfaces.
 30. The flame holder of claim 28 wherein the area in plan view of the one or more of the plurality of openings decreases from the bulging region to the outlet
 31. A flame holder, comprising: a refractory body defining a proximal side, a distal side spaced downstream from the proximal side, and a plurality of openings extending therethrough, each of the plurality of openings extending between an inlet on the proximal side and an outlet on the distal side, one or more of the plurality of openings including: a bulging region located downstream from the inlet thereof; and an area in plan view that decreases from the inlet to a narrowed region and increases from the narrowed region to a bulging region, the narrowed region positioned downstream from the inlet and upstream from the bulging region.
 32. The flame holder of claim 28 wherein the one or more of the plurality of openings exhibits at least one of a generally tapered shape or one or more curved inner surfaces. 